What Happens to Air Pressure as You Move From the Troposphere to the Thermosphere?

Pressure

• Air pressure is a measure of how much force the atmosphere applies, per unit area, to everything underneath it. If you could mark off a square, one foot on each side, and weigh all the air above that square, all the way to the end of the atmosphere, the result would be the pressure per square foot. As altitude increases, there is less air above any given point. The atmosphere is also not evenly distributed -- it is much thicker near the ground. As a result, pressure decreases quickly as altitude increases.

Pressure in the Troposphere

• At ground level, pressure in the troposphere is usually very near 1000 millibars, or approximately 1 atmosphere. At the top of the troposphere, at a line called the tropopause around 10 km in altitude, pressure is about a fifth of that: 200 millibars or 0.2 atmospheres.

Pressure in the Stratosphere

• The stratosphere begins just above the tropopause and ends at the stratopause, around 50 km in altitude. Pressure decreases steadily with altitude, from 200 millibars or 0.2 atmospheres at the bottom of the stratosphere to 1 millibar or 0.001 atmosphere as the top.

Pressure in the Mesosphere

• The mesosphere begins just above the stratopause and ends at the mesopause, around 85 km in altitude. Pressure decreases steadily with altitude, from 1 millibar or 0.001 atmospheres at the bottom of the mesosphere to 0.005 millibars or 0.000005 atmospheres at the mesopause.

Pressure in the Thermosphere

• The thermosphere is the largest layer of the atmosphere, beginning at the mesopause around 85 km and ending at 500 km in altitude, though the atmosphere gradually gets thinner and thinner so there is no exact boundary where the thermosphere ends. Pressure in the upper thermosphere is incredibly low, and air molecules are so far apart that sound can no longer be transmitted.